Regioselective synthesis of rapamycin derivatives

ABSTRACT

This invention provides a regioselective process for preparing a 42-ester or ether of rapamycin, and 31-silyl ether intermediates.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. Not Yet Known, which was converted from U.S. patentapplication No. 09/408,830, filed Sep. 29, 1999, pursuant to a petitionfiled under 37 C.F.R. 1.53(c)(2)(i).

BACKGROUND OF THE INVENTION

[0002] This invention relates to the regioselective synthesis ofderivatives of rapamycin at the 42-position, which are useful forinducing immunosuppression, and in the treatment of transplantationrejection, graft vs. host disease, autoimmune diseases, diseases ofinflammation, adult T-cell leukemia/lymphoma, solid tumors, fungalinfections, and hyperproliferative vascular disorders.

[0003] Rapamycin is a macrocyclic triene antibiotic produced byStreptomyces hygroscopicus, which was found to have antifungal activity,particularly against Candida albicans, both in vitro and in vivo [C.Vezina et al., J. Antibiot. 28, 721 (1975); S. N. Sehgal et al., J.Antibiot. 28, 727 (1975); H. A. Baker et al., J. Antibiot. 31, 539(1978); U.S. Pat. No. 3,929,992; and U.S. Pat. No. 3,993,749].

[0004] Rapamycin alone (U.S. Pat. No. 4,885,171) or in combination withpicibanil (U.S. Pat. No. 4,401,653) has been shown to have antitumoractivity. R. Martel et al. [Can. J. Physiol. Pharmacol. 55, 48 (1977)]disclosed that rapamycin is effective in the experimental allergicencephalomyelitis model, a model for multiple sclerosis; in the adjuvantarthritis model, a model for rheumatoid arthritis; and effectivelyinhibited the formation of IgE-like antibodies.

[0005] The immunosuppressive effects of rapamycin have been disclosed inFASEB 3, 3411 (1989). Cyclosporin A and FK-506, other macrocyclicmolecules, also have been shown to be effective as immunosuppressiveagents, therefore useful in preventing transplant rejection [FASEB 3,3411 (1989); FASEB 3, 5256 (1989); R. Y. Calne et al., Lancet 1183(1978); and U.S. Pat. No. 5,100,899].

[0006] Rapamycin has also been shown to be useful in preventing ortreating systemic lupus erythematosus [U.S. Pat. No. 5,078,999],pulmonary inflammation [U.S. Pat. No. 5,080,899], insulin dependentdiabetes mellitus [U.S. Pat. No. 5,321,009], smooth muscle cellproliferation and intimal thickening following vascular injury [U.S.Pat. No. 5,516,781], adult T-cell leukemia/lymphoma [European PatentApplication 525,960 A1], and ocular inflammation [U.S. Pat. No.5,387,589].

[0007] Numerous rapamycin 42-derivatives are known, typically beingesters (carbon and sulfur based) or ethers of the 42-hydroxyl group ofrapamycin, that are produced by esterification or etherification of the42-position. Esterification of rapamycin at the 42-position was commonlyprepared by directly reacting rapamycin with acylating agents in orderto afford the desired product. The chemistry appeared to be rathersimple. However, as rapamycin contains two secondary hydroxyl groups atpositions 31 and 42, attempts to discriminate between these twofunctional centers in order to achieve a selective synthesis of42-monoacylated product, posed a difficult challenge. This type ofnon-regioselective reaction also produced a 31,42-bis-acylatedby-product and as well, some unreacted rapamycin remained in thereaction mixture. The final result was a lower yield that requiredextensive purification to obtain pure 42-monoacylated product.

DESCRIPTION OF THE INVENTION

[0008] This invention provides a regioselective method for thepreparation of a 42-ester or ether rapamycin having the structure

[0009] wherein R is an ester or ether, which comprises:

[0010] (a) treating rapamycin with a silylating agent to form rapamycin31,42-bis-silyl ether;

[0011] (b) selectively hydrolyzing the 42-silyl ether in mild acid toprovide rapamycin 31-silyl ether;

[0012] (c) treating the rapamycin 31-silyl ether with a suitableesterifying or etherifying reagent to form rapamycin 31-silyl ether42-ester or ether; and

[0013] (d) selectively hydrolyzing the 31-silyl ether in mild acid toprovide the desired rapamycin 42-ester or ether.

[0014] Preferred 42-esters and ethers of rapamycin which can be preparedby the method provided by this invention are disclosed in the followingpatents, which are all hereby incorporated by reference: alkyl esters(U.S. Pat. No. 4,316,885); aminoalkyl esters (U.S. Pat. No. 4,650,803);fluorinated esters (U.S. Pat. No. 5,100,883); amide esters (U.S. Pat.No. 5,118,677); carbamate esters (U.S. Pat. No. 5,118,678); silyl ethers(U.S. Pat. No. 5,120,842); aminoesters (U.S. Pat. No. 5,130,307);acetals (U.S. Pat. No. 5,51,413); aminodiesters (U.S. Pat. No.5,162,333); sulfonate and sulfate esters (U.S. Pat. No. 5,177,203);esters (U.S. Pat. No. 5,221,670); alkoxyesters (U.S. Pat. No.5,233,036); O-aryl, -alkyl, -alkenyl, and -alkynyl ethers (U.S. Pat. No.5,258,389); carbonate esters (U.S. Pat. No. 5,260,300); arylcarbonyl andalkoxycarbonyl carbamates (U.S. Pat. No. 5,262,423); carbamates (U.S.Pat. No. 5,302,584); hydroxyesters (U.S. Pat. No. 5,362,718); hinderedesters (U.S. Pat. No. 5,385,908); heterocyclic esters (U.S. Pat. No.5,385,909); gem-disubstituted esters (U.S. Pat. No. 5,385,910); aminoalkanoic esters (U.S. Pat. No. 5,389,639); phosphorylcarbamate esters(U.S. Pat. No. 5,391,730); carbamate esters (U.S. Pat. No. 5,411,967);carbamate esters (U.S. Pat. No. 5,434,260); amidino carbamate esters(U.S. Pat. No. 5,463,048); carbamate esters (U.S. Pat. No. 5,480,988);carbamate esters (U.S. Pat. No. 5,480,989); carbamate esters (U.S. Pat.No. 5,489,680); hindered N-oxide esters (U.S. Pat. No. 5,491,231);biotin esters (U.S. Pat. No. 5,504,091); and O-alkyl ethers (U.S. Pat.No. 5,665,772). These patents also disclose methods for esterificationor etherification utilized in step (c), above.

[0015] The following scheme illustrates the regioselective preparationof rapamycin 42-ester with 2,2-bis-(hydroxymethyl)propionic acid, as arepresentative 42-ester of rapamycin, which can be prepared according tothe method provided in this invention. The original synthesis ofrapamycin 42-ester with 2,2-bis-(hydroxymethyl)propionic acid isdisclosed in U.S. Pat. No. 5,362,718.

[0016] While the chemical preparation of rapamycin esters and ethersappears to be simple, and the desired products are obtainable, thesynthetic yield of the esters and ethers is often poor. As rapamycincontains two secondary hydroxyl groups at positions 31 and 42, attemptsto discriminate between these two functional centers in order to achievea selective synthesis of 42-monoester such as compound [B] poses adifficult challenge. For example, the synthesis of rapamycin 42-esterwith 2,2-bis-(hydroxymethyl)propionic acid described in U.S. Pat. No.5,362,718, example 10, was non-regioselective, the 31,42-bisesterby-product was also generated. As a result, the crude product [B] afterwork-up contains the desired product [B], 31,42-bisester by-product andunreacted rapamycin. In an effort to consume the remaining startingrapamycin, the reaction was allowed to proceed for a longer period withnegative consequences, the quantity of the 31,42-bisester increasedsignificantly. The resulting crude product [B] is contaminated withunreacted rapamycin and 31,42-bisester, and subsequent columnchromatography purification effort has proved to be difficult as the42,31-bisester has a very close retention time with product [B].Overall, the major obstacle in large-scale production of compound [B]appears to be the non-regiospecificity that is further complicated bypurification difficulties. This invention overcomes these difficultiesby providing a regioselective synthesis of 42-esters or ethers ofrapamycin by selectively protecting the 31-hydroxyl group as a silylether (i.e., compound [D]), leaving the 42-position hydroxyl accessiblefor regioselective esterification or etherification to produce31-O-silyl, 42-esters or ethers (i.e., compound [E]). The 31-hydroxylgroup can then be deprotected under mild acidic conditions (i.e.,compound [B]).

[0017] In accordance with this invention, it is preferred that the 31,and 42-hydroxyl groups are protected as trialkyl silyl ethers. The42-silyl protected hydroxyl group of the 31,42-bis-silylated rapamycincan be selectively cleaved under mildly acidic conditions to provide31-silyl rapamycin. The silylating agents used for this transformationare common, commercially available chloroalkylsilanes, such aschlorotrimethylsilane, chlorotriethylsilane or chlorotripropylsilane.However, the bulkier the trialkylsilane, the more time is needed todeprotect in acid media during the penultimate chemical step toregenerate the 31-hydroxyl group. Also, a longer reaction time in theacid media generates more degradation by-products. Although,chlorotrimethylsilane, chlorotriethylsilane or chlorotripropylsilane canbe used for the preparation of rapamycin 31-O-trialkylsilyl ethers,chlorotrimethylsilane is the preferred silylating agent. Thetrimethylsilyl group is more acid labile and therefore easier tode-protect during the transformation and in effect, this minimizes theformation of degradation products. The preparation of31-O-trimethylsilyl rapamycin [D] is described in Example 1. Rapamycinis treated with excess chlorotrimethylsilane in ethyl acetate at 0-5° C.in the presence of an organic base and the 42- and 31- hydroxyl groupsof rapamycin are silylated to form rapamycin 31,42-bis-O-trimethylsilylether in quantitative yield. The common organic bases such as imidazole,1-methylimidazole, triethylamine and N,N-diisopropylethylamine can beused for the general silylation reaction. However, imidazole is found tobe the preferred base for the silylation of rapamycin as the reaction iscompleted within 30 minutes.

[0018] Selective de-protection of the 42-O-trimethylsilyl group ofrapamycin 31,42-bis-O-trimethylsilyl ether to form rapamycin31-O-trimethylsilyl ether, is effected in situ at 0-5° C. with ethanol,ethanol-water mixtures, water, diluted inorganic or organic acids.Sulfuric acid (0.5 N) is preferred since the reaction is clean and canbe completed in 4-5 h. A number of organic solvents can be used forsilylation and in particular, DMF is often mentioned in the literature.However, in this invention, ethyl acetate is the preferred solvent.

[0019] The esterification or etherification of the 31-protectedrapamycin can be carried out under conditions described in the patentslisted above. For example, in Scheme I, the acylation of rapamycin31-trimethylsilyl ether was accomplished using 2,4,6-trichlorobenzoylmixed anhydride of 2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid in thepresence of 4-dimethylaminopyridine or a similar reagent. In addition,2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid chloride was also foundto be an effective acylation agent in this invention in the presence of4-dimethylaminopyridine or a similar reagent. For the acylationconditions, methylene chloride is the preferred solvent rather thantetrahydrofuran which was described in the prior art. In addition, lowerreaction temperature of less than 0° C., with −20 to −15° C. or lowerbeing more preferred, provides better results than the room temperatureacylation described in U.S. Pat. No. 5,362,718. In Scheme I, theacylation products, 31-O-TMS, 42-(protected-hydroxy) esters (compound[E]) can be further treated with diluted acid to convert them to42-(protected-hydroxy) esters (compound [B]) or used directly to makefinal product 42-hydroxyesters (final product [C]). This methodology canbe used to prepare other esters or ethers of rapamycin, by simplyvarying the esterfiying or etherifying agent used.

[0020] In Scheme I, conversion of compound [B] to rapamycin 42-esterwith 2,2-bis-(hydroxymethyl)propionic acid [C], can be accomplishedunder mildly acidic conditions. It is preferred that aqueous sulfuricacid is used, as it minimizes the formation of impurities generated whenaqueous hydrochloric acid is used, as described in the U.S. Pat. No.5,362,718. The tetraene impurity formed when using hydrochloric acid hasbeen reported to be difficult to separate from the desired product bycolumn chromatography (Caufield et al, Tetrahedron Lett., 1994,37,6835). It is also preferrable to carry out the hydrolysis at 0-5° C.rather than room temperature as described in U.S. Pat. No. 5,362,718.

[0021] The synthetic route disclosed in this invention provides severaldistinct advantages over the synthetic methodology which has beenpublished for the preparation of rapamycin esters or ethers; mainly inthe yield and ease of purification of the desired 42-esters or ethers.As this is a regioselective synthesis, the overall yields of the desired42-esters or ethers is dramatically improved. For example, the syntheticmethodology taught in U.S. Pat. No. 5,362,718 provides compound [B] in a35% yield, whereas, the synthesis of [B] is accomplished in 85% yieldusing the methodology disclosed herein. Additionally, the conversion torapamycin 42-ester with 2,2-bis-(hydroxymethyl)propionic acid from [B]is accomplished in approximately 75% yield using the process describedherein, whereas only a 20% conversion is provided using the methodologyof U.S. Pat. No. 5,362,718.

[0022] Using the same methodology, 42-ethers of rapamycin can beprepared in a regioselective manner. As an example, U.S. Pat. No.5,665,772 discloses the preparation of 40-O-alkyl ethers of rapamycin ina non-regioselective manner. Owing to nomenclature differences, the42-position of rapamycin (as named in this invention) is referred to asthe 40-position in U.S. Pat. No. 5,665,772. These positons areidentical. Using the methodology disclosed herein, rapamycin31-O-trimethylsilyl ether can be treated with, for example,2-(t-butyldimethylsilyl)oxyethyl triflate) to provide31-O-trimethylsilyl, 42-O-[2-(t-butyldimethylsilyl)oxy]ethyl-rapamycin.Removal of the silyl protecting groups from the 31-hydroxyl group ofrapamycin and from the 42-hydroxyethyl moiety can be accomplished undermildly acidic conditions, such as dilute sulfuric acid to provide42-O-(2-hydroxy)ethyl rapamycin. The non-regioselective formation ofother 42-ethers of rapamaycin is disclosed in U.S. Pat. No. 5,665,772.These also can be prepared regioselectively via rapamycin 31-Otrimethylsilyl ether.

[0023] This invention also covers 31-silyl ethers of rapamycin, whichare useful in the preparation of the 42-esters and ethers of rapamycin,as disclosed herein. The silicon moiety as represented by —SiR′R″R′″,contains 3 groups which can be the same or different. Typical silylethers of this invention contain R′, R″, or R′″ moieties which are alkylof 1-6 carbon atoms, phenyl, or benzyl groups. The alkyl groups can bebranched or straight chain. It is preferred that R′, R″, and R′″ bealkyl groups, and more preferred that R′, R″, and R′″ are methyl orethyl. It is still more preferred that the 31-silyl ether is rapamycin31-O-trimethylsilyl ether.

[0024] The following examples illustrate the preparation of rapamycin31-silyl ethers and a rapamycin 42-ester, which is representative of thecompound which can be prepared by the process of this invention.

EXAMPLE 1 Rapamycin 31-O-trimethylsilyl ether

[0025] A solution of rapamycin (25.0 g, 92.4% strength; 25.28 mmol) in750 mL ethyl acetate was cooled to 0-5° C.; 7.5 g (110.20 mmol) ofimidazole was added and stirred to form a solution. To this coldsolution 11.0 g (101.25 mmol) of chlorotrimethylsilane was addeddropwise over 30 min and stirred for a further 30 min at 0-5° C. inorder to complete the formation of rapamycin 31,42-bis-O-trimethylsilylether. A 50 mL quantity of 0.5 N sulfuric acid was added dropwise over a10 min period and the mixture was stirred for 2.5 h at 0-5° C. Thereaction mixture was transferred into a separatory funnel and theaqueous layer was separated and extracted with 125 mL ethyl acetate. Theorganic layers were combined and successively washed with brine (125mL), saturated sodium bicarbonate solution (100 mL), water (125 mL×2)then brine to pH 6-7. The organic layer was dried over anhydrous sodiumsulfate and evaporated under reduced pressure to give a beige color foamproduct, 28.5 g (theory 24.94 g). HPLC analysis showed it contained 86%(by area %) of rapamycin 31-O-trimethylsilyl ether and 7% of rapamycin.The product was used directly for the subsequent reaction.

[0026] LC/MS electrospray (+) mode (M−H)=985. ¹H NMR (400 MHz, d-6 DMSO)δ 4.60 (m, 1H, (42C)OH), 4.10 (m, 1H, C(31) H), 3.09 (m, 1H, C(42) H),−0.027 (s, 9H, 31-O-TMS).

EXAMPLE 2 2,2,5-Trimethyl[1.3-dioxane]-5-carboxylic acid chloride

[0027] A solution of 2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid(17.42 g, 0.1 mol) in 200 mL of dry toluene was warmed to 40° C., and26.0 mL of oxalyl chloride (37.83 g, 0.3 mol) added dropwise over aperiod of 30 min and then stirred at 40° C. for 2.5 h. The reactionmixture was evaporated under reduced pressure to remove solvent andexcess oxalyl chloride. The residual product was evaporated twice withdry toluene (200 mL), then dried under high vacuum at 40° C. for 2 h toobtain 19.75 g of product as an orange colored liquid. ¹H NMR (300 MHz,CDCl₃) δ 4.28 (2H, d, J=10.5 Hz), 3.76 (2H, d, J=10.5 Hz), 1.46 (3H, s),1.29 (3H, s). ¹³C NMR (75 MHz, CDCl₃) δ 176.43, 98.76, 66.06, 52.07,25.82, 21.20, 18.10.

EXAMPLE 3 Rapamycin 31-O-trimethylsilyl ether, 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid

[0028] Method A:

[0029] To a solution of the 2,2,5-trimethyl[1.3-dioxane]-5-carboxylicacid (9.77 g, 56.08 mmol) and N,N-diisopropylethylamine (12.00 g, 92.80mmol) in 200 mL methylene chloride at room temperature under nitrogen,2,4,6-trichlorobenzoyl chloride (13.35 g, 54.73 mmol) was added and theresulting mixture was stirred for 5 h at room temperature. The reactionmixture was cooled to −20 to −15° C. and a solution of rapamycin31-O-trimethylsilyl ether (28.50 g, crude, made from 25.28 mmol ofrapamycin) in 120 mL methylene chloride was added. A solution of4-dimethylaminopyridine (11.68 g, 95.60 mmol) in 110 mL methylenechloride was added dropwise over a 2 h period. The reaction mixture wasfurther stirred for 16 h at −15 to −16° C. The reaction mixture wasquenched with 100 mL water and the organic layer was separated andwashed with 0.5 N sulfuric acid (180 mL), followed by brine (100 mL),saturated sodium bicarbonate solution (100 mL), water (100 mL×2), brine(100 mL) to pH 6-7. The organic layer was dried over anhydrous sodiumsulfate and evaporated under reduced pressure to afford the titlecompound (33.18 g) as a beige color foam.

[0030] LC/MS electrospray (+) mode (M+NH₄)=1160. ¹H NMR (400 MHz, d-6DMSO) δ 4.57 (m, 1H, C(42)H), 4.10 (m, 1H, C(31) H), 4.03 (d, 2H), 3.57(d, 2H), 1.34 (s, 3H), 1.24 (s, 3H), 1.13 (s, 3H), −0.023 (s, 9H,31-O-TMS)

[0031] Method B:

[0032] A solution of rapamycin 31-O-trimethylsilyl ether (11.00 g; from10.0 g of rapamycin; 11.15 mmol) in 120 mL methylene chloride,containing 2 mL of N,N,-dimethylformamide, was stirred under nitrogenand cooled to −15 ° C., 4-dimethylaminopyridine (4.80 g, 39.29 mmol) wasadded and the mixture was stirred to form a solution. To this coldsolution a 7.5% solution of 2,2,5-trimethyl[1.3-dioxane]-5-carboxylicacid chloride (42.18 g; 16.42 mmol) in methylene chloride was addeddropwise over a 2 h period. The solution was further stirred for 1 h at−15° C., and an additional 7.5% solution of acid chloride (14.06 g, 5.47mmol) in methylene chloride was added over a 30 min period. The reactionmixture was further stirred for 16 h at −15 C. to −16° C. The reactionmixture was quenched with 100 mL brine and the organic layer wasseparated and washed with cold 0.5 N sulfuric acid (100 mL), brine (100mL), saturated sodium bicarbonate solution (100 mL) water (100 mL),brine (100 mL) to pH 6-7. The organic layer was dried over anhydroussodium sulfate and evaporated under reduced pressure to afford product(12.15 g) as a yellow foam.

EXAMPLE 4 Rapamycin 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid

[0033] A solution of rapamycin 31-O-trimethylsilyl ether, 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid (33.18 g; from example 3,method A) in 100 mL of acetone was stirred and cooled to 0-5° C. To thiscold solution 17 mL of 0.5 N sulfuric acid was added dropwise over a 10min period and the mixture was stirred for 2.5 h at 0-5° C. A solutionof sodium bicarbonate (1.44 g) in 20 mL water was added over a period of20 min followed by an additional 33 mL water over a period of 30 min;the product started to precipitated after about 1 h of stirring. Themixture was stirred overnight at 0-5° C. and after filtration the solidproduct was washed with 60 mL of acetone-water (1:1). The product wasdried in a vacuum oven at 30° C. to obtain 28.85 g of product (83.9%strength, 89.3% overall yield from rapamycin). The ¹H NMR of the productwas identical to the product described in U.S. Pat. No. 5,362,718example 10.

EXAMPLE 5 Ragamycin 42-ester with 2,2-bis-(hydroxymethyl)propionic acid

[0034] Method A:

[0035] A solution of rapamycin 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid (28.85 g; from example 4)in 276 mL of tetrahydrofuran was stirred and cooled to 0-5° C. To thiscold solution 83 mL of cold 2 N sulfuric acid was added dropwise over a30 min period and the mixture was stirred for 60 h at 0-5° C. Thereaction mixture was diluted with 600 mL of ethyl acetate and washedwith 120 mL brine. The aqueous layer was extracted once with 120 mL ofethyl acetate and the organic extracts were combined and washed withsaturated sodium bicarbonate solution (120 mL), water (200 mL×2) andbrine (120 mL) to pH 6-7. The organic phase was dried over anhydroussodium sulfate and evaporated under reduced pressure at room temperatureto obtain product (28.42 g), as a beige color foam. The crude productwas chromatographed on a silica gel column and eluted with 30% acetonein heptane to give 18.06 g of pure product, a white solid (69.4% overallfrom rapamycin). The ¹H NMR of the product is identical to the productdescribed in U.S. Pat. No. 5,362,718 example 11.

[0036] Method B:

[0037] A solution of rapamycin 31-O-trimethylsilyl ether, 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid (23.25 g, prepared from20.06 g of rapamycin, strength 92.7%, 20.34 mmol) in 230 mL oftetrahydrofuran was stirred and cooled to 0-5° C. To this cold solution115 mL of cold 2 N sulfuric acid was added dropwise over a 45 min periodand the mixture was stirred for 88 h at 0-5° C. The reaction mixture wasdiluted with 500 mL of ethyl acetate and washed with 100 mL brine. Theaqueous layer was extracted once with 100 mL of ethyl acetate and theorganic extracts were combined and washed with saturated sodiumbicarbonate solution (80 mL), water (80 mL×2) and brine (100 mL) to pH6-7. The organic phase was dried over anhydrous sodium sulfate andevaporated under reduced pressure at room temperature to afford product(22.48 g), a beige color foam. The crude product was chromatographed ona silica gel column and eluted with 30% acetone in heptane to give 16.50g of pure product as a white solid (78.4% overall from rapamycin). The¹H NMR of the product is identical to the product described in U.S. Pat.No. 5,362,718 example 11.

EXAMPLE 6 Rapamycin 31,42-bis-O-trimethylsilyl ether

[0038] A solution of rapamycin (10.0 g, 94.3% strength; 10.3 mmol) in150 mL ethyl acetate was cooled to 0-5° C.; 3.0 g (44 mmol) of imidazolewas added and stirred to form a solution. To this cold solution, 4.4 g(40.5 mmol) of chlorotrimethylsilane was added dropwise over a 20 minperiod and following this, the solution was stirred at 0-5° C. for afurther 30 min. The reaction mixture was filtered to remove theimidazole HCl and the filtrate was evaporated under reduced pressure toobtain a yellow foam. Heptane (200 mL) was added and stirred at roomtemperature for 20 min and the mixture was filtered. The filtrate waswashed with 40 mL of saturated sodium bicarbonate solution, then twicewith water (80 mL), then brine (50 mL).

[0039] The organic layer was dried over anhydrous sodium sulfate andevaporated to obtain the product, a yellow foam of 11.42 g (98.6%).

[0040] LC/MS electrospray (−) mode (M−H)=1057. ¹H NMR (400 MHz, d-6DMSO) δ 4.10 (m, 1H, C(31) H), 3.31 (m, 1H, C(42) H), 0.057 (s, 9H,42-O-TMS), −0.027 (s, 9H, 31-O-TMS).

EXAMPLE 7 Rapamycin 31-O-triethylsilyl ether

[0041] A solution of rapamycin (5.00 g, 92.7% strength; 5.07 mmol) in 75mL ethyl acetate was cooled to 0-5° C.; 1.50 g (22.03 mmol) of imidazolewas added and stirred to form a solution. To this cold solution, 3.05 g(20.23 mmol) of chlorotriethylsilane was added dropwise over a 10minutes period. The mixture was stirred for 30 min at 0-5° C., thenstirred at room temperature overnight to complete the formation ofrapamycin 31,42-bis-O-triethylsilyl ether. Following filtration of thereaction mixture, the filtrate was evaporated under reduced pressure atroom temperature to remove most of the solvent. The residual solution(ca. 10 mL) was dissolved in 60 mL acetone and 15 mL of 0.15 N sulfuricacid was added and the mixture stirred for 25 h at 0-5° C. The rapamycin31,42-bis-O-triethylsilyl ether disappeared at this stage. The reactionmixture was diluted with 80 mL of ethyl acetate and successively washedwith brine (60 mL×2), saturated sodium bicarbonate solution (40 mL),water (60 mL×2), brine (60 mL) to pH 6-7. The organic layer was driedover anhydrous sodium sulfate and evaporated under reduced pressure toobtain a product of light yellow gum, 6.92 g (theory 5.21 g). HPLCanalysis showed it contained 95.2% (by area %) rapamycin31-O-triethylsilyl ether and 0.9% rapamycin.

EXAMPLE 8 Rapamycin 31-O-tripropylsilyl ether

[0042] A solution of rapamycin (5.00 g, 92.7% strength; 5.07 mmol) in 75mL ethyl acetate was cooled to 0-5° C.; 1.50 g (22.03 mmol) of imidazolewas added and stirred to form a solution. To this cold solution, 3.91 g(20.3 mmol) of chlorotripropylsilane was added dropwise over 10 minperiod. The mixture was stirred for 30 min at 0-5° C., then at roomtemperature for 21 h to complete the formation of rapamycin31,42-bis-O-tripropylsilyl ether. Following filtration of the reactionmixture, the filtrate was evaporated under reduced pressure at roomtemperature to remove most of the solvent and the residual solution wasdissolved in 60 mL acetone. A 15 mL quantity of 0.25 N of sulfuric acidwas added and the mixture was stirred for 45 h at 0-5° C.; the rapamycin31,42-bis-O-tripropylsilyl ether disappeared at this stage. The reactionmixture was diluted with 100 mL of ethyl acetate, and successivelywashed with brine (40 mL×2), saturated sodium bicarbonate solution (40mL), water (40 mL×2), and brine (50 mL) to pH 6-7. The organic layer wasdried over anhydrous sodium sulfate and evaporated under reducedpressure to obtain a product of light yellow gum, 8.07 g (theory 5.43g). HPLC analysis showed it contained 96.7% (by area %) of rapamycin31-O-tripropylsilyl ether and 1% of rapamycin

What is claimed is:
 1. A process for preparing a 42-ester or ether ofrapamycin having the structure

wherein R is an ester or ether, which comprises: (a) treating rapamycinwith a silylating agent to form rapamycin 31,42-bis-silyl ether; (b)selectively hydrolyzing the 42-silyl ether in mild acid to providerapamycin 31-silyl ether; (c) treating the rapamycin 31-silyl ether witha suitable esterifying or etherifying reagent to form rapamycin 31-silylether 42-ester or ether; and (d) selectively hydrolyzing the 31-silylether in mild acid to provide the desired rapamycin 42-ester or ether.2. The process according to claim 1 , wherein the silylating agent instep (a) is a trialkylsilyl halide.
 3. The process according to claim 2, wherein the silylating agent in step (a) is trimethylsilyl chloride.4. The process according to claim 3 , wherein the acid used to performthe hydrolysis in step (b) is dilute sulfuric acid.
 5. A process forpreparing rapamycin 31-trimethylsilyl ether, which comprises: (a)treating rapamycin with chlorotrimethylsilane in an inert solvent in thepresence of a suitable base to provide rapamycin31,42-bis-trimethylsilyl ether; and (b) treating the31,42-bis-trimethylsilyl ether with dilute acid to provide rapamycin31-trimethylsilyl ether.
 6. The process according to claim 5 , whereinthe base in step (a) is imidazole, 1-methylimidazole, triethylamine, orN,N-diisopropylethylamine.
 7. The process according to claim 6 , whereinthe acid in step (b) is sulfuric acid.
 8. A process for preparingrapamycin 42-ester with 2,2-bis-(hydroxymethyl)propionic acid, whichcomprises: (a) treating rapamycin with a silylating agent to formrapamycin 31,42-bis-silyl ether; (b) selectively hydrolyzing the42-silyl ether in mild acid to provide rapamycin 31-silyl ether; (c)acylating the rapamycin 31-silyl ether with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid chloride or the2,4,6-trichlorobenzoyl mixed anhydride of2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid to giverapamycin-31-O-trimethylsilyl ether, 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid; (d) selectivelyhydrolyzing the 31-silyl ether in mild dilute acid to provide rapamycin42-ester with 2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid; and (e)treatment of rapamycin 42-ester with2,2,5-trimethyl[1.3-dioxane]-5-carboxylic acid with mild acid to provide42-ester with 2,2-bis-(hydroxymethyl)propionic acid.
 9. The processaccording to claim 8 , wherein the silylating agent is a trialkylsilylhalide.
 10. The process according to claim 9 , wherein the silylatingagent is chlorotrimethylsilane.
 11. The process according to claim 10 ,wherein the acid used in steps (b) and (d) is sulfuric acid.
 12. Theprocess according to claim 11 , wherein the acylation in step (c) iscarried out at less than 0° C.
 13. A process for preparing42-O-(2-hydroxy)ethyl-rapamycin which comprises: (a) treating rapamycinwith a silylating agent to form rapamycin 31,42-bis-silyl ether; (b)selectively hydrolyzing the 42-silyl ether in mild acid to providerapamycin 31-silyl ether; (c) treating the rapamycin 31-silyl ether witha an ethylene glycol equivalent containing an acid labile hydroxylprotecting group protecting on one terminus of the ethylene glycolequivalent and a leaving group suitable of alkylating a hydrxyl group asthe other terminus of the ethylene glycol equivalent. (d) hydrolying theprotecting groups on the 31-postion and on the 42-hydroxyethyl positionunder mildly acidic conditions.
 14. The process according to claim 13 ,wherein the silylating agent is is a trialkylsilyl halide.
 15. Theprocess according to claim 14 , wherein the silylating agent ischlorotrimethylsilane.
 16. The process according to claim 15 , whereinthe ethylene glycol equivalent is 2-(t-butyldimethylsilyl)oxyethyltriflate).
 17. The process according to claim 16 , wherein the acid usedin steps (b) and (d) is sulfuric acid.
 18. A compound which is arapamycin 31-O-silyl ether.
 19. The compound of claim 18 , in which the31-O-silyl ether is a trialkylsilyl ether.
 20. The compound of claim 19, which is rapamycin 31-O-trimethylsilyl ether.